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Gain Factor vs Gain

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#1 jsowens

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Posted 15 March 2019 - 03:34 AM

I recently revamped my 'sub length' calculation and now use the "(Signal / Gain factor) + Bias" formula

 

As I read the threads here on CN, I see that gain also plays an important role when striving for the optimal dynamic range.  Like most things this varies based on the local LP.

 

I'd like to experiment with this but I'm not sure how (or if) gain is adjustable on the atik460ex.....does anyone know?

 

I know it has a 'gain factor' of .27e-/ADU but how to set to 50/100 based on sky darkness?

 

 

Thanks

J



#2 Noah4x4

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Posted 15 March 2019 - 05:05 AM

The Atik 460ex is a CCD camera. I believe that most CCD cameras have a fixed Gain setting within a range of 65,536 discrete settings that is only adjustable by a potentiometer prior set at the factory (but I am unsure as I don't own a CCD)? If that fact is confirmed by CCD owners, I suspect that answers the OP question? Gain can't be adjusted.

 

My further understanding is because 65,536 is greater then the full well setting of a typical camera with (say) 20,000e there is little value in having adjustable Gain. I am sure a more knowledgeable expert will comment if I am wrong. However, and sorry to potentially hijack the thread, but I do have a very similar question relating to my Atik Horizon CMOS that might be relevant to the OP in the event that we discover that Gain is adjustable in his Atik 460ex. I suspect that we have the same motivation to pursue this having recently read about the benefits of adjustable Gain in the context of 'darkness' and 'light pollution' etc. My Atik Horizon, being CMOS, does have adjustable Gain.

 

But in SG Pro, when using the Atik ASCOM Driver, there are these Gain settings (see below) for 'Value' and 'e~/ADU'.

 

ADU.jpg

 

<Link Cam> cannot be adjusted. But I can  change the numbers into 'e~/ADU' (n.b. ignore the 5 shown in this image, I was just experimenting!).

 

I know that my camera has a full well of 20,000e and that at 1v produces 4096 ADU which is then amplified x 16 to 65,536 ADU (e.g. mirroring the optimum potential ADU of a CCD). I have tried various calculations using these numbers in the e~/ADU column and it doesn't seem to make a blind bit of difference. My histogram is always stuck far left with minimal adjustment available.  Yet in Atik Infinity I seemingly have a full range of adjustment. However, I always get a good quality image on screen in SG Pro despite having negligible histogram adjustment available. I am begining to wonder whether this lack of Gain adjustment is because all other Atik cameras are CCD and hence this adjustable gain functionality has perhaps been omitted from the Atik ASCOM driver?  It might not even be deemed necessary give the Horizon is designed to mimic CCD (given its 65,536 ADU at 20,000e)? I have a current support request into Atik, but have had no reply. Any ideas folks?

 

I am currently playing with other options - first setting Gain in Atik 'Infinity' then switching to SG Pro where I then benefit from autofocus, 'darks on the fly' etc etc. In Infinity there are three presets; Low; Medium High that I understand represent 1x, 5x and 30x Gain. I know that 30x Gain is pretty useless as most images will just saturate and at 5x (medium) is the "Unity Gain". To be frank, I have enjoyed my best results with Low x1 (which are excellent). At the Unity Gain (x5), 65,536 ADU is reached at around half of the full well. I am not expert, but I think that explains the saturation at quite low to medium levels of Gain. The camera produces remarkably high (amplified) ADU and is hence very sensitive at surprisingly low levels of electrons. My calculations using the amplified ADU numbers in an Atik Video all seem to suggests an 'e~/ADU' number of around 0.06 to 0.3. The S G Pro default is 0.00, and these are trivial variances from that. So, I am starting to conclude (rightly or wrongly!) that I might as well forget these settings and leave them at the default 0.00. I also read somewhere that the ZWO ASI1600 had a maximum ADU of 4096 at 20,000e. I wonder, is this 16x amplification why the Horizon was more expensive than the ZWO with the same sensor? I am proceeding using trial and error from here, but if there is a photography expert that can make more sense of this than me then his/her contribution to clarify things is most welcome.


Edited by Noah4x4, 15 March 2019 - 05:09 AM.


#3 Noah4x4

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Posted 15 March 2019 - 02:25 PM

This issue has now been fixed for SG Pro and Maxim. Well done Atik for a very prompt response.

 

Atik has released a new ASCOM driver which means you can select  a specific Atik camera and if CMOS you can now select the Gain pre-set (Low, Medium, High) to apply via SG Pro camera settings. Then  e~/ADU defaults to 0.4 corresponding with the upper end of my calculations above .

 

There still seems to be an issue with Sharpcap, so I suggest Sharpcap users don't yet upgrade to the latest driver, as the old worked (but Gain didn't) when the new errors. I have reported to Atik and Sharpcap support.



#4 Jon Rista

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Posted 15 March 2019 - 02:47 PM

The Atik 460ex is a CCD camera. I believe that most CCD cameras have a fixed Gain setting within a range of 65,536 discrete settings that is only adjustable by a potentiometer prior set at the factory (but I am unsure as I don't own a CCD)? If that fact is confirmed by CCD owners, I suspect that answers the OP question? Gain can't be adjusted.

The gain on a CCD is definitely not always optimally set to fill the 16-bit range. A lot of CCD cameras have two gain settings as well, a high gain and a low gain mode. Depending on the setting, one gain setting may more than fill the 16-bit range, meaning if you just barely saturate the wells, you'll clip in the digital data by more. The other setting may not fill the 16-bit range.

 

It is best to check CCD documentation to figure out what the actual gain in e-/ADU is.


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#5 jsowens

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Posted 15 March 2019 - 03:38 PM

Does the Atik 460EX have such a setting.....high/low/anything?

Does anybody know?

Thanks



#6 Jon Rista

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Posted 15 March 2019 - 03:49 PM

Does the Atik 460EX have such a setting.....high/low/anything?

Does anybody know?

Thanks

Doesn't even matter if there IS a setting. The key is, the gain is NOT always tuned to optimally fill the 16-bit range. So saturating the FWC may underfill or clip when actually converted, depends on the camera. Check the documentation for all CCDs.



#7 jsowens

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Posted 16 March 2019 - 12:20 PM

Thanks Jon but the reason I ask is that if it is somehow adjustable I'd like to know so I can test some lower settings to improve Dynamic Range or higher settings for Narrowband, etc




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